SC1404ITSTR datasheet(12/27 Pages) SEMTECH

12 / 27 page

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SC1404

PRELIMINARY

POWER MANAGEMENT

Applications Information

where Vin_nom=15V, Vo=3.3V, D=Vo/Vin_nom, Fs=300KHz,

Ts=3.33uS and

∆∆∆∆∆Io=2.2 A. Lo is then calculated to be 3.9uH. For

the interest of this design, Lo is chosen to be 4.7uH for the 3.3V

output. For the 5V output, a coupled inductor with 6.4uH primary

(5V winding) inductance is used.

Choosing Current Sense Resistor

Since the SC1404 implements Virtual Current SenseTM, an exter-

nal current sense resistor is not needed by the control loop. But it

is required for cycle-by-cycle current limit. Cycle-by-cycle current

limit is reached when the voltage across the current sense resis-

tor exceeds 50mV nominal. Depending on the system require-

ment, this current limit can vary, it is usually 10 to 30% higher than

the maximum load. Taking into consideration lower limit of the

50mV, the value of the current sense resistor can be calculated

using the following equation:

For a DC OC trip point between 8 to 12A, Rsense is chosen to be

5m Ω .

Choosing the Main Switching MOSFET

Before choosing the main switch MOSFET, we need to know two

critical parameters: voltage and current rating. In order to mini-

mize the conduction loss, we recommend using the lowest Rds(on)

for the same voltage and current rating. The maximum drain to

source voltage of the switch MOSFET is mainly decided by the

topology of the switcher. Since this is a buck topology,

Applying a derating of 70%, a 30V MOSFET is used in the design.

The peak current of the MOSFET is determined by

The following calculations are done to verify that the power dissi-

pation of the main switch MOSFET is well within 1.86W, which is

the maximum allowable power dissipation for the package.

where Rds(on) = 0.01 Ω @Tj=25 C

° and Vgs = 4.5V. In order to

find Rds(on)@ Tj=100 C

° , use 1.40*Rds(on)@25 C

° . Therefore,

Rds(on) @ Tj = 100 C

° is equal to 0.014 Ω .

where

= 7.1A,

= 4.9A and

The worst case conduction loss is calculated to be 25mW. And the

switching loss of the MOSFET is given by,

where Crss is the reverse transfer capacitance of the MOSFET; it

is equal to 200pF for STS12NF30L, Ig is the gate driver current; it

is equal to 1A for SC1404. And Vin_nom = 15V, fs = 300KHz. The

switching loss is calculated to 81mW. And the gate loss is given by,

where Cg=11nF, V=5V and fs=300KHz. The gate loss is calcu-

lated to be 41mW.

So the total power dissipation is calculated to be 147mW and is

well within the maximum power dissipation allowance of the

MOSFET. No special heating sinking is required when laying out

the MOSFET.

According to the calculated voltage and current rating, Si4886DY,

IRF7413, FDS9412 or STS12NF30L meets the requirement. The

specs for these MOSFETs are listed in the table below. For the

purpose of this exercise, STS12NF30L is chosen. Next step is to

determine its power handling capability. Based on 85 C

° ambient

temperature, 150 C

° junction temperature and 50 C

° /W ther-

mal resistance, its power handling is calculated as follows:

J = 150°C;

A = 85°C;

= 50°C/W

)

(

)

(

)

(

GATE

⋅

nom

RMS

)

(

⋅

GATE

SWITCHING

CONDUCTION

DISS

TOTAL

nom

RMS

)

(

CONDUCTION

⋅

OUT

RSS

SWITCHING

⋅

NOM

OUT

nom

MAX

∆

−

1.30W

150

−

IPEAK

Ω

(min)

SENSE

MAX

∆

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